Electrostatographic printing operates according to well known principles such as electrographic printing in which an electrostatic charge is deposited image-wise on a dielectric recording member. According to another technique, referred to as Direct Electrostatic Printing (DEP) as described in EP-A-0 675 417, toner particles are imagewise deposited on a substrate without the use of an electrostatic latent image. A cloud of toner particles is supplied to a mesh of apertures, where each aperture has its own control electrode to control the amount of toner propelled through the aperture onto a substrate on which the image must be formed. Electrostatographic printing also includes electrophotographic printing in which an overall electrostatically charged photoconductive dielectric recording member is image-wise exposed to conductivity increasing radiation producing thereby a "direct" or "reversal" toner-developable charge pattern on said recording member. The toner image is transferred onto a printing stock material, usually paper or a synthetic material such as PET (polyethyleneterephthalate) in the form of a web whereon the toner image is fixed, whereupon the web is cut into sheets containing the desired print frame. As can be learned from the book "The Physics and Technology of Xerographic Processes" by E. M. Williams (1984), Chapter Ten, p. 204 et seq the transfer of developed toner images onto paper proceeds by means of electrical corona devices to generate the required electric field to attract the charged toner from the electrostatographic recording member to the paper. The transfer efficiency of toner onto the receptor paper or synthetic material is not only dictated by the contact of the paper with the toner-laden recording member and the deposited charge but also by the conductivity of the recording member and particularly by its water content. Moreover, the conductivity may be highly dependent on the type of receptor material, i.e. the difference of conductivity between paper and e.g. PET may be very important. Paper is not a simple insulating dielectric, so the electrical properties of plain paper have some influence on toner transfer. Experiments with a variety of paper types and thicknesses (i.e. weights) have established that heavier papers yield improvement in transfer efficiency. Paper types with high porosity, i.e. high permeability for gases loaded with ions by corona discharge do not allow an efficient toner transfer. Variation in gas permeability or porosity between different paper types is due to overall thickness, degree of filling with clays, sizings, and other paper treating substances. Apart from the paper fibres and said substances which form a constant factor for conductivity or volume resistivity there is the moisture content which fluctuates with the humidity of the environment, especially the environment of the paper storage unit containing the paper on roll. It has been established that as the moisture content increases from about 3 to 10% by weight, the surface resistance of copy paper decreases nearly six orders in magnitude. Dry paper has very good electric insulating behaviour so that thereon by corona discharge a fairly high electrostatic charge can be deposited before breakdown takes place. On using dry receptor paper the toner attraction force caused by said electrostatic charge can be built up with a reasonable corona charge. Since the leakage of charges through the receptor paper is a function of moisture content (paper humidity), a careful control of said moisture content will be in favour of toner transfer efficiency, image quality and reproducibility in toner printing results. A system for control of the moisture content has been proposed in EP-A- 0 629 925. That system includes a heating means, a cooling means and at least one electrometer to derive the moisture content from the electrical conductivity of the paper web and for correcting the heating means on that evaluation of the moisture content. Tests have shown, however, that the measured electrical conductivity is not only a function of the moisture content alone. As a consequence, the heating means may further influence or increase the temperature of the moving paper web in an attempt to lower the moisture content, whereas that moisture content already reached an optimum value. This shows that a problem remains to be solved. Even if other types of sensors are used to assess the moisture content of the media, such as humidity sensors or direct contact resistor sensors, other parameters still influence the relation between the measured value and the absolute moisture content.